There are many circumstances under which the manual assembly of electronic circuits may be necessary or desirable. This is especially true of low volume operations where automated assembly equipment is unavailable or not economically justified. Such circumstances may include low volume manufacturing, prototyping, repair, rework or revision of electronic circuits.
In recent years there has been a trend toward the use of smaller and smaller electronic components. This has been especially true since the introduction of the technology of surface mounted devices. While these new technologies present significant economies in high volume automated production, they have made the hand assembly of circuits more difficult and time consuming and therefore more costly. Moreover these smaller components require more careful and accurate placement on the circuit board that is difficult to achieve by hand assembly. In addition, direct manual manipulation of these smaller devices may cause unacceptable mechanical deformation of the leads or contamination of the contact points which would interfere with their attachment by solder or adhesives.
A number of implements have been devised to aid in the manual assembly of electronic circuits. Most of these implements can be categorized as either hand tools or assembly workstations.
The hand tool category consists mostly of grasping devices to aid in the holding and manipulation of the electronic components. Simple hand tools such as pliers or tweezers allow the user to more easily hold small components without direct manual contact. This effectively eliminates the problems of contamination associated with direct hand contact, but it does little to improve the accuracy of component placement. In fact, much of the time these tools actually reduce the accuracy of placement because any small movement of the hand is amplified by the length of the tool. Orientation of the component and correct alignment of the contact pins or pads on the component with the pads on the circuit board is still difficult with the hand tools. It often involves a tedious trial and error process to correctly place the component on the circuit board.
Another group of hand tools involves vacuum operated grasping devices. Using a vacuum to hold the components effectively reduces contamination and component damage. However, it does not solve the problems of accuracy since it still depends on the accuracy and steadiness of the user's hand for placement of the components. These devices also introduce another complication associated with the release mechanism. Most vacuum graspers have a finger-operated release mechanism on the device. Operating the release can cause movement of the tool at the precise moment of release, undoing the operator's painstaking efforts to accurately place the component. Using a foot switch to operate the vacuum release avoids the problem of hand of movement during release, but it introduces an imprecise delay in the release timing which can adversely affect placement accuracy.
Assembly workstations are another approach for assisting the hand assembly of printed circuit boards. Workstations generally have a device for holding the printed circuit board and a device for assisting placement of the electronic components. Some workstations may also include other functions such as component storage, vision enhancements and soldering, heating or other process capabilities. At the very high end, the workstations may incorporate servo position controls, and even programmable assembly controllers, making them essentially into an automated assembly robot.
The simplest of these workstations assist the hand placement of electronic components. The printed circuit board is held in a rack which adjusts to the size of the board. The operator's forearm rests on an aluminum plate which moves in the x-direction on rails above the circuit board. The operator uses hand tools such as a vacuum gripper to place the electronic components on the board while his or her hand is steadied by the aluminum armrest. There is no control of the .theta. orientation of the components. While this approach facilitates repeated movement by organizing the workplace, the contribution to improving accuracy is minimal.
A more sophisticated manual workstation from OK Industries, eliminates the need for the operator to handle the components. The printed circuit board is held in a rack which adjusts to the size of the board. Components are stored in special component bins built into the workstation. A built-in vacuum gripper moves on rails in the x- and y-directions above the circuit board. The x- and y-positions of the gripper are controlled by hand and the z-position and the .theta.-rotation are manually controlled through servo mechanisms. This allows for placement of electronic components under manual control, but still depends on the operator's hand for steadiness and accuracy.
Positioning tables are another approach to workstations where the circuit board moves on an x-y table beneath a stationary component holder. This approach has the advantage of high accuracy, but it is not well suited to aiding hand assembly as positioning tables are generally large and expensive and they obscure the work from view impeding manual control.
Within the range of workstations there are many tradeoffs between placement accuracy, expense, speed, ease of use and space required. In general, they are very expensive because they must be built with painstaking precision to maintain close tolerances of component positioning over the entire surface of the PCB. Often, special training is needed to operate the workstation. These combined factors often work together to force a facility to get by with a single workstation which must serve for all assembly and repair operations. This causes inconsistencies and inefficiencies in the work flow.
Though they are more effective than hand tools, workstations like these are also much larger and more expensive. This leaves a large gap between the inexpensive, but ineffective, hand tools and the expensive workstations. Very few devices have been proposed to fill this gap to provide an accurate, but lower cost method for placing components on a printed circuit board. U.S. Pat. No. 4,956,911 to Zaremba and Bayer proposes a tool for repairing surface mounted components on printed circuit boards. This device has a pair of L-shaped legs that rest on the printed circuit board and a vacuum mechanism on a plunger that moves in the z-direction. The position of the component is adjusted by contact with locating cams mount on the legs of the device. This device has limitations in that it provides adjustment of the component position only for the z-axis and .theta.-angle. The x- and y-positions can only be corrected by moving the entire device, and there are no means to adjust the .alpha.- and .beta.-angles to assure that the component and the printed circuit board are coplanar.